As well known, 9% Ni steel is high-tensile steel used at ultralow temperature of about −196° C., and has high yield strength and excellent low-temperature toughness. The 9% Ni steel is therefore widely used, as ultralow-temperature steel, for storage tanks for, for example, LNG, liquid nitrogen, and liquid oxygen, or various types of associated equipment thereof. To effectively use such excellent ultralow-temperature toughness of the 9% Ni steel, a weld metal of a weld joint formed by welding between 9% Ni steel pieces is naturally required to have substantially the same properties of, such as ultralow-temperature toughness, as those of the base metal.
Under such circumstances, various investigations have been made on a technique for welding between 9% Ni steel pieces. For example, it is expected that when a welding wire (so-called similar-metal-composition welding wire) having a composition equal or similar to that of 9% Ni steel base metal for ultralow-temperature service is used for welding of the 9% Ni steel, a weld joint having excellent ultralow-temperature property is produced. A welding process such as MIG welding that is more efficient than TIG welding, however, cannot secure stable low-temperature toughness. Since MIG welding has such a difficulty in low-temperature toughness, the welding process is limited to TIG welding despite its lower welding efficiency, resulting in extremely low operation efficiency. Hence, the similar-metal-composition wire has been substantially not used.
FIG. 1 illustrates an example of a flash butt weld joint (welding test example) between 9% Ni steel plates. In either TIG welding or MIG welding, a weld metal 3 of a weld joint 1a between 9% Ni steel plates 2a and 2b is formed through the same process of sequential formation of deposition beads (1) to (13) in a multilayered manner with high heat input. In FIG. 1, a symbol 5 indicates a backing metal.
In TIG welding, a relatively small amount of deposition is made so that a thin bead is produced; hence, for example, a bead (12) is completely reversely transformed by, for example, a subsequent bead (13). As a result, a relatively coarse unaffected zone (solidified structure) of each deposition layer is transformed into a fine reheated structure. Specifically, a structure of a lower layer is refined due to an appropriate heat treatment effect caused by a heat cycle during welding of an upper layer, leading to improvement in low-temperature toughness of the lower layer.
In contrast, in highly efficient MIG welding, a relatively large amount of deposition is made, which therefore necessarily results in alternate disposition of the reheated structures and the unreheated, unaffected zones (solidified structures) in a thickness direction. As a result, the weld metal 3 of the joint 1a formed by MIG welding is difficult to have stable low-temperature toughness compared with that formed by TIG welding.
Hence, a welding wire of a Ni-based alloy (so-called Inconel) having the Ni content of as high as about 60% has been mainly used for welding of 9% Ni steel by highly efficient MIG welding. The weld joint formed with such a Ni-based alloy welding wire exhibits excellent toughness even at −196° C. in the as-welded condition, which is however extremely low in tensile strength, particularly in 0.2% yield strength, compared with a 9% Ni steel base metal. As a result, although 9% Ni steel as high-tensile steel is used, design stress must be reduced due to the low strength of the weld joint. Consequently, thickness of a welded structure as a whole must be disadvantageously increased to secure certain strength of the weld joint.
Hence, as long as the Ni-based alloy welding wire is used, the high strength of 9% Ni steel is not sufficiently used, which inevitably leads to multiple loads or burdens such as an increase in thickness and in weight of a welded structure, and an increase in consumption of the expensive Ni-based alloy welding wire. In addition, if welding is performed using the Ni-based alloy welding wire, there inevitably occurs a problem of hot crack associated with Ni, and a problem of thermal fatigue that is caused by a difference in thermal expansion coefficient between the Ni-based alloy welding wire and the 9% Ni steel base metal during welding due to a significant difference in composition therebetween.
Although 9% Ni steel itself has the excellent properties of the ultralow-temperature steel as described above, applicability thereof has been in fact extremely limited due to the above-described limitations in welding operation.
Thus, a study has been made on a welding technique using a similar-metal-composition welding wire, which has a composition equal or similar to that of the 9% Ni steel base metal, in place of the Ni-based alloy welding wire in order to improve the ultralow-temperature property of the weld joint using the similar-metal-composition welding wire.
For example, PTL1 discloses a method of improving the ultralow-temperature property by adjusting and/or limiting, within an appropriate range, the content of each of nickel, manganese, boron, and oxygen in a chemical composition of the similar-metal-composition welding wire for 9% Ni steel. PTL1 reports results of improvement in low-temperature toughness of a weld joint evaluated by the Charpy impact test in accordance with JIS-Z-3111, which is however evaluated only in terms of the entire absorbed energy. In other words, no investigation is made in terms of crack initiation strength.
PTL2 proposes a method of improving low-temperature toughness of a weld joint through designing a welding operation process using the similar-metal-composition welding wire for 9% Ni steel. Specifically, PTL2 discloses a method where the surface of a final-layer weld bead in multilayer welding is cooled to 150° C. or less, and then the surface of the final-layer weld bead is remelted by arc from a non-consumable electrode while being shielded by an inert gas. This method is intended to improve the low-temperature toughness by applying heat treatment to the final layer in a groove center through remelting of the final layer that is less likely to receive a heat treatment effect due to the heat cycle during welding of an upper layer. This method, however, causes a problem of an increase in number of steps in welding operation, and merely improves the low-temperature toughness of only the final welding layer as a part of the weld joint. Hence, low-temperature toughness of the entire weld metal, which dominates the properties of the weld joint, is naturally limitedly improved.
PTL3 proposes a technique for improving low-temperature toughness through control of carbide morphology and decrease of heat treatment time of a weld bead portion in welding using the similar-metal-composition welding wire for 9% Ni steel. In this technique, a similar-metal-composition welding wire is used in Example, where the welding wire contains 0.042% or more REM (Rare Earth Metal) added thereto though there is no description is made on the reason why REM is added. The technique also causes an increase in number of steps due to the heat treatment required after welding as in PTL2, which in turn causes an increase in cost. Moreover, wire compositions are not sufficiently investigated. In addition, the low temperature toughness required in light of the crack-initiation resistance strength is also not considered. REM refers to Rare Earth Metal, and is a general term of elements of La to Lu in the periodic table.
While such improvements in low-temperature toughness have been made using the similar-metal-composition welding wire for 9% Ni steel, they are in common lacking in the viewpoint of elucidation from crack initiation, i.e., the viewpoint of crack-initiation resistance strength reflecting actual crack initiation. Hence, the low-temperature toughness required for actual structures has not been evaluated in detail though sufficient low-temperature toughness satisfying a certain criterion is achieved in evaluation of absorbed energy by a Charpy impact test or a COD test for evaluation of typical low-temperature toughness.
On the other hand, techniques for improving the low-temperature toughness by the similar-metal-composition welding wire for 9% Ni steel are quite recently proposed, for example, in PTL4 and PTL5, which also include development of the evaluation method itself of the low-temperature toughness from the viewpoint of the crack-initiation resistance strength.
As disclosed in PTL4 and PTL5, when external force (a load) is actually applied to a welded structure such as the storage tank for, for example, LNG, liquid nitrogen, and liquid oxygen, or associated equipment thereof, a crack is initiated and then propagated. To evaluate the low-temperature toughness required based on crack-initiation resistance strength reflecting such actual crack initiation, it is indispensable to measure the toughness from the start to the end of a crack initiation during application of the external force.
A test method of ultralow-temperature toughness, achieving such measurement, includes an instrumented Charpy impact test method, which provides a load displacement curve that allows separation between crack initiation and a crack propagation process during a Charpy impact test. This measurement method allows measurement of a toughness value (absorbed energy) Ei at crack initiation under application of external force, measurement of a toughness value (absorbed energy) Ep during crack propagation from the start to the end of a crack, and measurement of the crack-initiation resistance strength (the maximum load). The total toughness value Et (Ei+Ep) of Ei and Ep and the above-described crack-initiation resistance strength enable more detailed evaluation of ultralow-temperature toughness in correspondence to large-size brittle fracture strength of an actual welded structure.
From such a viewpoint, the technology of PTL4 improves the ultralow-temperature toughness particularly by adding Cr (chromium). Specifically, during multilayer welding by TIG welding, a lower layer in the center of a weld metal in a groove is subjected to a heat treatment effect caused by a heat cycle during previous welding of an upper layer. During this process, if an initial structure, which has been transformed into a bainite or martensite structure after welding, of the lower bead in the center of the weld metal is reversely transformed into austenite by the heat treatment effect, the structure of the weld metal is easily refined. In a composition system of the 9% Ni steel containing Ni and Mn, Cr has an excellent function of reducing the ferrite-austenite transformation temperature. In the technology of PTL4, a specified amount of Cr is contained in the similar-metal-composition wire to use such a unique property of Cr, which allows the structure of the weld metal to be refined, resulting in improvement in crack-initiation resistance strength of the weld joint.
Similarly, the technology of PTL5 improves the ultralow-temperature toughness particularly by adding REM. In general, oxides extremely degrade the low-temperature toughness; hence, it is not preferable to form a large number of large oxides in the weld metal of the joint between 9% Ni steel pieces. If, however, an oxide, which is formed through a reaction with a small amount of oxygen in the weld metal, is sufficiently small, such an oxide does not serve as an initiation site of fracture, but rather advantageously serves as a pinning grain that inhibits crystal grain growth during or after solidification of weld. Consequently, a fine oxide effectively improves strength and toughness of the entire weld metal at low temperature.
In the technology of PTL5, REM is determined to be optimum as an element having such an effect, and a specified amount of REM is contained in the similar-metal-composition welding wire to disperse an appropriate amount of fine oxides of REM in the weld metal. The REM oxides each have a property of good wettability to a melted iron alloy compared with other metal oxides, for example, Al oxides. As a result, even if the REM oxides are formed in a liquid phase of the weld metal, the oxides are less likely to agglomerate and thus do not grow into larger grains. This allows the REM oxides to be left as fine grains, and thus each of the oxides serves as the pinning grain that inhibits the crystal grain growth during or after solidification of weld. Consequently, the REM oxides effectively improve strength and toughness of the entire weld metal at low temperature.
In addition, PTL6 proposes a technology that allows a specified amount of REM to be contained in the similar-metal-composition welding wire. In the technology of PTL6, REM and Ga are added in combination, and a ratio of REM amount to Ca amount is controlled to be within a fixed range to stabilize arc in pure Ar gas.